Devices of that type are already known, and they present various drawbacks to which the present invention seeks to provide a solution that is advantageous from technical and economic points of view.
German utility certificate DE-GM 7914263 describes a single-pole current transformer of that type. The core support tube is put into place in appropriate recesses formed in two end plates. An insulating ring is associated with one plate to avoid making an electrical connection between the two plates via the support tube. A peripheral cylindrical tube surrounding the core is placed between the two plates which are also coupled together by means of clamping screws. By adjusting the length of the support tube and of the peripheral cylindrical tube it is possible in simple manner to make different transformers provided with various numbers of cores of different lengths.
Nevertheless, the outside diameter of the transformer is necessarily greater than that of the largest core so as to leave a gap for passing the clamping screws. Given that the diameter of the transformer is crucial in determining the space occupied by multi-pole switchgear, since it defines the spacing between the transformers corresponding to respective phases, the solution recommended is not advantageous because it does not enable the dimensions of the switchgear to be reduced.
European patent application published under the number EP 0 665 561 A1 describes a single-pole current transformer having an annular core in which the metal enclosure is made up of two castings. One of the castings constitutes a base plate and the other casting constitutes the side walls of the metal enclosure and also its top closure plate. Each of the two parts has a peripheral belt, these belts being organized to be juxtaposed and partially engaged and clamped one against the other by bolts. The belts are of diameter greater than the diameter of the core.
To modify the length of the current transformer, it is necessary to modify the length of the two castings accordingly. This means that for each type of current transformer, it is essential to make two molds corresponding respectively to the two castings of different lengths.